The conventional color CRT such as used in a television receiver or computer terminal incorporates a shadow mask having a large number of electron beam passing apertures. The shadow mask is sometimes referred to as a color selection electrode because it restricts the position of the electron beams incident upon the CRT's display screen to only selected phosphor deposits on the inner surface of the screen to provide the desired color for a video image presented thereon. The typical shadow mask is in the form of a thin metal foil and is maintained in either a curved or flat configuration. The flat configuration for the shadow mask is used in high resolution CRTs as employed in high definition television receivers. In the typical color CRT, the electron beam passing apertures are either in the form of elongated, vertically aligned slots or circular apertures in the shadow mask. It is the shadow mask with circular beam passing apertures with which the present invention is concerned.
A typical shadow mask contains hundreds of thousands of the aforementioned beam passing apertures in a hexagonal array with center-to-center aperture spacing of less than 1 mm. Corresponding to each beam passing aperture is a triad of red, green and blue emitting phosphor dots, also less than 1 mm in diameter, which are approximately tangent to each other and are disposed on the inner surface of the CRT's display screen, or glass faceplate. The apertures of the shadow mask are used not only to restrict access of the electron beams to only designated phosphor dots on the CRT's display screen during CRT operation, but are also used in forming the phosphor dots. Using a "lighthouse" principle, light from a single source is directed through the apertured shadow mask onto the phosphor coated inner surface of the display screen during CRT manufacture. The incident light, which is typically at an ultraviolet (UV) frequency, hardens a photosensitive binder in the phosphorescent coating leaving a large number of triad phosphor dots on the display screen after the remaining phosphor material is washed away.
It is important both during formation of the CRT's phosphor display screen as well as during CRT operation that the circular beam passing apertures in the shadow mask have a circular cross-section. Non-circular apertures give rise to misaligned phosphor dots and electron beam landing errors which reduce video image brightness and degrade video image color purity. The problem of non-circular beam passing apertures is most severe near the four corners of the generally rectangular mask because of the increased tension applied to these areas. In these areas, the tension is applied generally in two transverse directions causing the apertures and corresponding phosphor dots on the display screen to be elongated, or oval, in shape. Attempts to compensate for these non-circular apertures employing complicated electron focusing lens designs and phosphor exposure arrangements have met with only limited success in improving video image brightness and color purity.
The present invention addresses the aforementioned limitations of the prior art by providing an apertured shadow mask for use in a color CRT which initially has oval beam passing apertures in its corner areas, but which assume a circular cross-section when the shadow mask is stretched during installation in the color CRT. The shadow mask thus affords circular electron beam passing apertures over its entire surface.